Download Meiosis

Meiosis
What now?
• We talked about mitosis which is how 99% of
your cells divide
• What about your gametes? Do they divide?
• Yes, they do!
• Meiosis – the process of forming haploid
gametes from diploid cells
• Haploid – have half of the chromosomes
• Diploid – have a full set of chromosomes
Meiosis
• Meiosis has two phases: Phase 1 and Phase 2
• The stages are the same as mitosis now they
are followed by 1 and 2
• Example: Prophase 1, Metaphase 1,
Anaphase 1, Telophase 1, Cytokinesis,
Prophase 2, Metaphase 2, Anaphase 2,
Telophase 2 and Cytokinesis
• There is no stop at interphase so the DNA
doesn’t replicate again!!!
Meiosis 1
• Prophase 1 – the first stage of meiosis 1
• A. The nuclear membrane breaks down
• B. The centrioles move from the center of the
cell to opposite ends
• C. Sister chromatids pair up in the cells
• D. Crossing over occurs because the
chromosomes are close together
Metaphase 1 / Anaphase 1
• Metaphase 1 – the chromosomes line up in
the middle of the cell
• Anaphase 1 – the chromosomes separate and
move to the opposite ends of the cell
Telophase 1
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Again – the opposite of Prophase 1
A. The spindles break down
B. Chromosomes uncoil and form two nuclei
C. The cell divides
Cytokinesis occurs BUT no interphase occurs
It goes straight to Prophase 2
Prophase 2
• Prophase 2 is the same as Prophase 1
• A. Chromosomes condense
• B. Spindles form in each cell and attach to the
chromosomes
• Nuclear Membrane breaks down
• Crossing over doesn’t occur this time
Metaphase 2 / Anaphase 2
• Metaphase 2 – the chromosomes line up in
the center of the cell
• Anaphase 2 – the chromosomes split apart
and move to opposite poles
Telophase 2
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Telophase 2 – opposite of prophase
A. Nuclear membranes reform
B. Spindles break down
C. Cells divide
Cytokinesis occurs and splits into 4 separate
cells
The Purpose of Meiosis
•To create 4 new cells
from 1 cell. Each new
cell has ½ of the
chromosomes as the
first cell.
Compare the Two…
Mitosis
• Happens in all cells
• Creates 2 cells
• Cells are diploid
• Humans – have 46
chromosomes per cell
Meiosis
• Happens in sex cells
• Creates 4 cells
• Cells are haploid
• Humans – have 23
chromosomes per cell